The present invention relates to a selective gas separation membrane and more particularly to a highly selective gas separation membrane formed of a polymer having fumaric diester units and suitable for separating and concentrating a specific component from a gaseous mixture.
It has long been known that a specific component can be separated and concentrated from a gaseous mixture by using a membrane formed of a polymeric material. This has recently attracted considerable attention from the standpoint of resources saving and energy saving. Particularly, if oxygen-enriched air with a high oxygen concentration can be obtained from air easily, inexpensively and continuously according to the membrane separation method, it is of great value. At present, the oxygen used for medical purposes such as the use thereof for incubators for immature infants or for therapy of patients suffering from diseases of the respiratory system is a pure oxygen filled in a cylinder. However, serious drawbacks are encountered in the use of such oxygen; for example, a troublesome operation is required and the oxygen cannot be fed continuously and must be diluted before use. But, if a highly efficient membrane capable of feeding oxygen in a concentrated state from air is obtained, the above drawbacks will be overcome, and it will become possible to use such oxygen even at home with the aid of an apparatus of a simple structure, and thus a great improvement can be expected in the medical field.
Further, if oxygen-enriched air can be fed easily and continuously by using a membrane also in various combustion systems presently in use such as, for example, industrial boilers and furnaces, iron household heaters, then the fuel consumption can be reduced at a higher combustion efficiency, that is, energy saving can be attained, and at the same time the problem of environmental pollution caused by incomplete combustion can be overcome. Moreover, if oxygen-enriched air can be fed easily and inexpensively by the use of a membrane, a further development can be expected also in other fields such as the food industry, aquaculture and waste disposal.
Polymeric materials presently known are more or less gas permeable, but in order to obtain an oxygen enriching membrane employable industrially, such materials must permit a sufficiently high permeation speed of oxygen and exhibit a large selectivity for oxygen relative to nitrogen. It has been known that the gas permeation speed is proportional to the permeation coefficient (usually represented by P in the unit of cm.sup.3 (STP)cm/cm.sup.2.sec.cmHg) peculiar to each polymeric substance, a differential pressure between both sides of a membrane and the surface area of the membrane and is inversely proportional to the membrane thickness. The selectivity for oxygen relative to nitrogen is determined by the ratio (PO.sub.2 /PN.sub.2) of the oxygen permeation coefficient (PO.sub.2) to the nitrogen permeation coefficient (PN.sub.2) which values are peculiar to each polymeric substance. Therefore, in order to obtain a practical permeation speed, it is necessary to select a material having a large PO.sub.2, or else it will become necessary to enlarge the differential pressure or the membrane surface area, thus resulting in increased size and complicated structure of the apparatus which employs the membrane. Moreover, in order to obtain a sufficient oxygen concentration, it is necessary to select a polymeric material having a high PO.sub.2 /PN.sub.2 ratio. Further, the membrane thickness must be reduced in order to attain as high a permeation speed as possible, and to this end the material strength must be high.
Thus, oxygen enriching membrane materials employable industrially are required to have large PO.sub.2 and PO.sub.2 /PN.sub.2 values and be superior in strength and durability.
However, there is scarcely any known polymeric substance that satisfies the above requirements. Although a number of attempts have been made for improving known high polymers, none of them have fully attained the purpose. The following table shows examples of PO.sub.2 and PO.sub.2 /PN.sub.2 of known high polymers.
______________________________________ PO.sub.2 (cm.sup.3 (STP)cm/ Polymeric Material cm.sup.2 .multidot. s .multidot. cmHg) PO.sub.2 /PN.sub.2 ______________________________________ Polydimethylsiloxane 3.5 .times. 10.sup.-8 1.9 Poly-4-methylpentene-1 3.0 .times. 10.sup.-9 2.9 Natural rubber 2.3 .times. 10.sup.-9 2.4 Low density polyethylene .sup. 2.9 .times. 10.sup.-10 2.9 Cellulose acetate .sup. 4.3 .times. 10.sup.-11 3.0 ______________________________________
There are only an extremely limited number of known high polymers that have PO.sub.2 values not smaller than 10.sup.-9. Examples are merely polydimethylsiloxane, poly-4-methylpentene-1 and natural rubber. Most of other known high polymers are of PO.sub.2 values not larger than 10.sup.-10, so it is impossible for these high polymers to afford a membrane having a practical permeation speed unless the membrane area is made extremely large. Besides, the PO.sub.2 /PN.sub.2 values of these high polymers are about 3 or so at the highest, and as the PO.sub.2 value increases, the PO.sub.2 /PN.sub.2 value decreases. Even known high polymers having PO.sub.2 values not smaller than 10.sup.-9 are still unsatisfactory for the use being considered. For example, natural rubber has a small PO.sub.2 /PN.sub.2 value and is poor in durability (especially in oxidation stability) because it has --C.dbd.C-- double bond in the main chain, which is a serious drawback. Poly-4-methyl-1-pentene exhibits a high strength, but has the drawback that its PO.sub.2 /PN.sub.2 value is small. Polydimethylsiloxane has the highest gas permeability among the high polymers presently known, but since its strength is poor, it is extremely difficult to form a membrane below 20 .mu. in thickness, and thus a practical permeation speed is not attainable despite of a large PO.sub.2 of the material. Besides, its PO.sub. 2 /PN.sub.2 value of 1.9 is the smallest, and thus even polydimethylsiloxane is not employable at all for obtaining a highly oxygen-enriched air.
Thus, there is scarcely known any high polymer having a practical PO.sub.2 value not smaller than 10.sup.-9, a high strength and a sufficiently large PO.sub.2 /PN.sub.2 value. Under the circumstances, it has keenly been desired to develop a new material which satisfies these requirements.